This invention relates to a method and apparatus for pressurizing closed, sealed articles, for example, tennis balls. It is important in the playing of tennis to use balls which are of uniform and regular characteristics. Among the many of such characteristics are the resilience and size of the ball which are partly dependent on the internal pressure of the ball. For example, a lower internal ball pressure will reduce the bounce of the ball while a higher pressure will increase its bounce and also may tend to enlarge the diameter of the ball slightly, but noticeably, particularly when in play. Changes from the desired internal pressure (approximately 15 p.s.i.) can result from a number of causes, perhaps the most common being repetitive use. Additionally, changes in atmospheric conditions (e.g., pressure or temperature) also can affect these characteristics of the ball. And where there is little change in atmospheric condition, prolonged shelving of the ball, even in stable atmospheric conditions, tends to result in a ball with reduced bounce and less liveliness.
Typically, tennis balls are packaged at the factory in hermetically sealed cans under pressure in an effort to increase the shelf life of the balls and to maintain them in a "lively condition" until ready for use. Even then, however, once the can has been opened and the pressure released, the useful life of the ball is relatively short, particularly after the balls have been used for a few games.
The most common difficulty encountered with tennis balls is that they lose their liveliness because of reduced internal pressure or weakening of the wall of the ball. There also are instances in which the ball has too much internal pressure, for example, as when the ball is brought to a location which is at a relatively high altitude. There the reduced atmospheric pressure results in an increased pressure differential between the inside and the outside of the ball which will tend to increase the liveliness beyond that which is normally expected and will also tend to increase the size of the ball slightly which has a noticeable effect on the response of the ball in play.